There are known mechanical switches that respond to changes in the switch environment, in fact such condition responsive mechanical switches are in wide use. An example is a pressure responsive switch, and a representative application is a pressure responsive switch associated with an aircraft turbine engine. For maximum fuel efficiency, the flow of coolant may be adjusted based on absolute pressure, such as by a solenoid valve controlled by a pressure responsive mechanical switch. Mechanical pressure switches can directly switch electrical leads using only two wires by being connected in series between a power source and the load. Improvements have been made to increase the useful life of mechanical switches, such as the use of sealed contacts. Nevertheless, contact fretting, wear, fatigue, and arcing in harsh environments have continued to be problems. In addition, in the case of a short circuit in the line closed by the switch, no effective current limit is provided.
Solid-state switches have no wearout or cycle life within the rated usage. However, for a system built or designed for use of a mechanical switch, substitution of a solid-state switch may not be possible without extensive redesign. In some fields such redesign can greatly increase the expense, such as in the aircraft industry where significant redesign of circuitry associated with an aircraft engine may require recertification. Such redesign could involve adding a third wire from airframe power to provide a continuous power supply current path for the electronic circuit of the solid-state switch. By comparison, mechanical switches typically require no external power.
Alternatively, the solid-state switch could be designed to have a continuous voltage drop across the switch sufficient to operate its own electronic circuitry. Nevertheless, the voltage drop would subtract from the supply voltage and thereby cause operational problems at low-voltage conditions. In addition, the several volt drop across the switch required to operate its internal circuitry, combined with a large load current, could result in several watts of power dissipation, which further complicates the situation because of excessive switch heating. In an aircraft turbine engine environment, heat already may be a problem given the extreme temperatures to which the switch is subjected.